Electrostatic transducers require an insulating structure/layer between their electrodes. In general, this insulating structure/layer has two main functions: it provides electrical isolation so an electrical charge can build up and it gives mechanical support. In state-of-the-art transducers, such as capacitive micromachined ultrasonic transducers (CMUTs), this insulating structure/layer is the main limiting factor in terms of device operation at high temperatures. The likelihood of an electrical breakdown of the insulation structure is dramatically increased at elevated temperatures. Further, parasitic capacitance in the insulation layer will significantly degrade the transducer sensitivity at elevated temperatures due to an increased mobility of trapped charges, for example alkali ions in silicon dioxide, and bias-temperature (BT) stress. Another main limiting factor for high-temperature operation are thermal-expansion related stress effects, when the electrostatic transducer is exposed to high temperatures.
Current fabrication methods leave the CMUT cell unprotected during the fabrication process, resulting in contamination of the cell cavity. Current CMUT cells do not that have thick insulation layers that are independent from the cavity height, which limits their high temperature and performance.
Accordingly, there is a need to develop a CMUT structure and fabrication method that reduces the number of fabrications steps, improves CMUT cell structure by increasing electrical breakdown voltage and reducing parasitic capacitance, provides better design flexibility and high controllability for the whole frequency range in which CMUT cells can be used (1 kHz-300 MHz). What is further needed is a CMUT cell that is completely protected mechanically and protected against contaminations by the membrane silicon-on-insulator (SOI) wafer in an early stage of the fabrication.